Conventional flame-retarding techniques for endowing combustible materials, such as wood-based materials, synthetic resins, inorganic salt molded articles (e.g., plasterboard and calcium silicate board), and paper with resistance against ignition at a relatively low temperature in the initial stage of fire, e.g., 600.degree. C. or lower, include (a) pressure-vacuum impregnation of fire-retarding chemicals into wood-based materials, (b) coating of a combustible material with a fire-retardant coating which foams on heating to form a heat insulating layer, and (c) laminating or lining of an incombustible inorganic layer on a combustible material.
The impregnation technique (a) is effective to prevent evolution of combustible gases from combustible materials on heating or selectively accelerate dehydration and carbonization but is, in turn, accompanied by considerable reductions in weight and volume. When continuously heated at, for example, about 1000.degree. C., burn-through is rather accelerated under some conditions.
According to technique (b), the foamed heat-insulating layer exhibits excellent fire retardance in the initial stage of fire, i.e., at relatively low temperatures. However, since the fire-retardant coating must generally be expanded to 100 fold or more so as to produce sufficient heat insulation effects, the foamed layer is liable to remove partially or over the entire surface when continuously heated at around 1000.degree. C. due to insufficient oxidation resistance and insufficient heat stream resistance.
Technique (c) is also very effective in the initial stage of fire at relatively low temperatures. When continuously heated at around 1000.degree. C., however, the incombustible layer unavoidably undergoes cracking or bursting due to a difference in thermal expansion coefficient between the inorganic layer and the combustible material or due to a pressure increase resulting from vaporization of a water content. Therefore, satisfactory fire-resistance cannot be obtained without increasing the thickness and weight of the incombustible layer or increasing the number of layers to be laminated.
Thus, there has not yet been established a technique which makes a thin material (e.g., 15 mm thick plywood) fire-resistant to such a degree as having a burn-through time of at least 30 minutes in a fire test at a simulated temperature after flashover (JIS A-1304, ASTM E-119, DIN-4102, etc.) or which makes a thick material (e.g., a 30 to 40 mm thick wood-based material) fire-resistant to such a degree as having a burn-through time of at least 1 hour or thermal deformation resistance while retaining characteristics of wood-based materials, such as heat retaining properties, lightweight, high specific strength, aesthetic appeal, moisture absorption and desorption, workability, and the like.
Being incombustible of themselves, inorganic materials such as plasterboards, calcium silicate boards and concrete show incombustibility in the above-described initial stage of fire. However, they essentially suffer from bursting or cracking when continuously heated at around 1000.degree. C. due to an increased vapor pressure of decomposed crystal water. A countermeasure which has generally been taken to avoid this is to thicken the material or to laminate a number of layers to thereby impart fire-resistance as desired. An attempt to make the material thinner by applying an intumescent fire-resistant coating or by laminating an intumescent fire-resistant mat has never been put to practical use.
Further, steel frames used as a structure of high buildings are essentially incombustible having a melting point exceeding 1500.degree. C. However, when continuously heated at about 660.degree. C. or higher under gravity or load, they suffers from an abrupt fall of strength, causing deformation or buckling. Fire-resisting of steel frames has generally been conducted through a hand method at the job site comprising winding a reinforcing net around the frames and spray coating thick with mortar containing incombustible fiber. In this field, too, application of an intumescent fire-resistant coating or laminating of an intumescent fire-resistant mat has not been established.